Quadrature phase generators are widely used for a variety of modulation and demodulation schemes in various wireless communication devices. Some of these schemes include Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) and the like. The quadrature phase generator takes a local oscillator (LO) signal and generates a pair of quadrature phase signals typically referred to as in-phase signal I and quadrature phase signal Q. The signals I and Q have frequency related to the LO frequency and have a phase difference of 90 degrees.
The design and topology of quadrature phase generators, along with the transmitter and receiver architectures within which they are used, vary for different communication devices. Several designs, such as divide-by-N flip flop circuits (where N is an even integer) and polyphase filters, exist. However, due to internal integrated circuit design and device tolerances, signal paths for the I and Q signals may have different propagation delays. As a result, the I and Q signals do not have an ideal quadrature phase difference of 90 degrees at the outputs of the quadrature phase generator. Any variation from the ideal quadrature phase difference of 90 degrees at the outputs of the quadrature phase generator is defined as a relative phase error between the I and Q signals.
FIG. 1 is an example illustrating error prone quadrature phase signal pairs ([I, Q] and [Ix, Qx]) generated by a typical quadrature phase generator. For the illustrated example, two error prone quadrature signal pairs should ideally be differentially related, i.e. in-phase signals I (0 degrees) 110 and Ix (180 degrees) 130, and the quadrature phase signals Q (90 degrees) 120 and Qx (270 degrees) 140 should have a phase difference of 180 degrees. However, due to the different propagation delays, the in-phase signals I 110 and Ix 130 have phase of 2 degrees and 179 degrees, and the quadrature phase signals Q 120 and Qx 140 have phase of 88 degrees and 271 degrees. Hence, the signals I 110 and Q 120 have a phase difference of 86 degrees (a relative phase error of −4 degrees) and the signals Ix 130 and Qx 140 have a phase difference of 92 degrees (a relative phase error of 2 degrees). The relative phase error for quadrature phase signal pairs subsequently results in errors in transmitting and receiving signals in a communication device.
FIG. 2 is a block diagram illustrating a prior art communication device 200 utilizing quadrature phase generators 220, 270 for a transmitter 210 and a receiver 260. The transmitter 210 utilizes transmitter (TX) quadrature phase generator 220 for modulating data 231, 233 to be transmitted. The receiver 260 utilizes receiver (RX) quadrature phase generator 270 for demodulating a received signal, for instance, a prefiltered signal 285.
In the transmitter 210, pre-amplifiers 230, 232 amplify the data 231 and 233 respectively to provide amplified signals 241, 243. Subsequently, TX filters 240, 242 pre-filter the amplified signals 241, 243 to provide bandlimited I/Q baseband signals 251, 253 to mixers 250, 252 respectively. The TX quadrature phase generator 220 takes LO signal 205 and provides I signal 221 and Q signal 223. The LO I signal 221 is provided to mixer 250 while the LO Q signal 223 is provided to mixer 252. Mixer 250 up-converts the bandlimited I signal 251 with the LO I signal 221 to provide RF signals 255. Mixer 252 up-converts the bandlimited Q signal 253 with the LO Q signal 223 to provide RF signal 257. Ideally, RF signals 255, 257 are phase shifted 90 degrees, but in reality these signals are prone to error. A combiner 254 combines the RF signals 255, 257 to provide a modulated signal 259 to a TX antenna (not shown) for transmission.
In the receiver 260, a RX antenna (not shown) receives a signal 281 or 283. A switch 282 switches between the signals 281, 283 based on a desired band for demodulation. A pre-filter 280 filters a signal (281 or 283) from the switch 282 to provide a prefiltered signal 285 to a pre-amplifier 286. The pre-amplifier 286 subsequently amplifies the prefiltered signal 285 to provide signals 287, 289 to mixers 290, 292 respectively. The RX quadrature phase generator 270 takes the LO input 205 and provides I signal 271 and Q signal 273 which are respectively provided to mixers 290, 292. Mixer 290 down converts signal 287 with I signal 271, while mixer 292 down converts signal 289 with Q signal 273. Ideally, the mixers 290 and 292 generate baseband signals 291, 293 with a phase difference of 90 degrees, but in reality these signals are prone to error. RX filters 298, 299 filter the baseband signals 291, 293 to provide demodulated baseband signals I 295 and Q 297.
Due to the relative phase error between the signal I 221 and the signal Q 223, the modulated signal 259 generated by the transmitter 210 contains an undesirable sideband image. Similarly, the relative phase error between the signal I 271 and the signal Q 273 results in an undesirable sideband image in the demodulated baseband signals 295, 297 generated by the receiver 260. Subsequently, the undesirable sideband images for the receiver and the transmitter may result in severe errors in detection of data during digital demodulation (receiver) or modulation (transmitter).
Common approaches for avoiding the problems associated with the relative phase error include modifying the phase for the baseband signals (data 231, 233) to be transmitted to match the phase difference between the signals I 221 and Q 223. Similarly, the received signal 285 is down-converted to the demodulated baseband signals 295, 297 to detect relative phase error. The phase for signals 295, 297 are adjusted based on the detected relative phase error. However, as the baseband signals for the transmitted and received signals have different relative phase errors, a separate quadrature phase generator is required for the transmitter 210 and the receiver 260. Circuit complexity, parts count, board area, power consumption, controller and logic complexity, and cost are major challenges for the communication devices using the aforementioned approaches.
Other approaches for addressing the problem of relative phase error in quadrature phase generators have been suggested. Approaches require a phase detector and several additional components such as filters, and operational amplifiers, and integrators in a feedback path to provide a phase adjustment signal for adjusting the phase of the I and Q signals. Again, circuit complexity and parts count are major concerns for such approaches. Furthermore, these approaches require a separate phase adjustment signal for each signal generated by the quadrature phase generator which adds to the complexity of the control circuitry used in the quadrature phase generator.
Accordingly, it would be desirable to have a method and apparatus capable of generating I and Q signals without the aforementioned issues.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.